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Geographic Information Systems. GIS Data Models. 1. Components of Geographic Data. Spatial locations Attributes Topology Time. Components – Spatial Locations. Specified with reference to a common coordinate system points lines polygons volumes grids.
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Geographic Information Systems GIS Data Models
1. Components of Geographic Data • Spatial locations • Attributes • Topology • Time
Components – Spatial Locations • Specified with reference to a common coordinate system points lines polygons volumes grids
Components - Attributes • Nominal - qualitative, e.g., grasses, trees, buildings, • Ordinal - rankble quality, e.g., high, medium, low • Interval - quantitative but not ratio, e.g., 400F, 800F • Ratio - absolute quantities, e.g., p, e, -96.345, …
Components • Topology Spatial relationship between geographic features Adjacency Containment Connectivity etc. • Time Presently treated as an attribute
2. Data Models • Real world entities and spatial entities
Data Models – Objects and Fields • Objects and Field View • Objects - Are discrete or have identifiable boundaries - Have attributes • Field - Varies continuously - Has no obvious boundaries
Data Models • Vector data model • Raster data model courtesy: Mary Ruvane, http://ils.unc.edu/
Data Models - (1) Vector • Every position has a pair of coordinates. • Lines and polygons are constructed by connecting a series of points. • Points, lines, and polygons are used to represent geographic features.
Epidemiological Studies- Disease Tracking Incidence of Viruses ESRI, GE SmallWorld
Positive Snail Prediction of Xichang Study Site using the land-cover map and field snail survey including 93 positive snail sites and 800 non-positive snail sites Positive snail 0.00 - 0.03 0.04 - 0.13 0.14 - 0.23 0.24 - 0.33 0.34 - 0.60 Lake ± 0 1,650 3,300 6,600 Meters
Data Models - (2) Raster • A matrix consists of regular grid cells • Positions are defined by column and row numbers • Each cell has a single value
Data Models - (2) Raster • Data consists of mostly attributes • A header file stores (1) Number of rows and columns, (2) Cell size, and (3) Coordinates of the origin • Coordinates of a location can be converted on the fly
2 Data Model(3) Advantages and Disadvantages • Raster • Vector
Ad and Disad - Raster • Advantages - It is a simple data model - Overlay operation can be easily implemented - High spatial variation is efficiently represented • Disadvantages - It is less compact - Topological relationships are difficult to represent - The output is less aesthetically pleasing
Ad and Disad - Vector • Advantages - It is a compact data model - It is efficient in coding topology - The output closely approximate hand-drawn maps • Disadvantages - It is a complex data model - Overlay operations are difficult to implement - The representation of high variation is inefficient
Readings • Chapter 3
Geographic Information Systems GIS Data Structures
GIS Data Structures • Raster data structures - Run length - Quadtrees • Vector data structures - Non-topological structure - Topological structure
3 Raster Data Structures(1) Run Length • Data are recorded in the order of Attribute1, number of cells in the run; Attribute2, number of cells in the run; …
3 Raster (1) Run Length 11,4; 3,4; 11,4; 3,4; 11,4; 3,4; 11,4; 3,4; 11,3; 3,1; 5,4 …
3 Raster Data Structures(2) Quadtrees • Subdividing a region into quadrants until each quadrant contains only one class - variable resolution
4 Vector Data Structures(1) Non-topological Structure • There is no topology and shared boundaries are recorded twice
4 Vector (1) Non-Topological Structure Storing coordinates of two polygons: and Polygon x1, y1; x2,y2; x3,y3; …. x1,y1. Polygon x1, y1; x2,y2; x3,y3; …. x1,y1. A (x1, y1) (x1, y1) 1 2 1 2 B (x2, y2) (x2, y2) 1 2
4 Vector (2) Topological Structure • No node or line segment is duplicated • Line segments and nodes can be referenced to more than one polygons • All polygons have unique identifiers • Island and hole polygons can be uniquely represented
4 Vector (2) Topological Structure 1 2 Storing coordinates of two polygons: and Line 1 x1, y1; x2,y2; x3, y3; x4, y4; x5,y5; Line 2 x1, y1; x2,y2; Line 3 x1, y1; x2,y2; x3, y3; x4, y4; Polygon Line 1, Line 2 Polygon Line 2, Line 3 (x1, y1) (x1, y1) Line 1 1 Line 2 Line 3 2 (x2, y2) (x2, y2) 1 2
4 Vector (2) Topological Structure • Polygon = a list of lines • Line = a set of coordinates
Topological- the Arc-Node Model • Arc - a line starting and ending at a node • Node - intersection point where two or more arcs meet or it is a dead end • Polygon - a closed chain of arcs
Arc-Node Model Polygon topology table Node topology table Arc topology table Arc coordinate data table
Readings Chapter 3